I would suggest to keep your idea "open" until say the first of next month.

It has already been mentioned that there are a lot of members that are off on trips right now.
Also, such as life is these days, folks might not be able to check the site more than once a week. And they just might not have anything to say/post then.

Might I suggest one amendment to your idea, it might generate more responses. List the one to be given away. Current economic times may prevent some from taking a chance, even for just the cost of shipping. OR, at least, a list of the possible ones to be given away. So folks can have an idea of which ones are in play. You don't even have to list all that you have in mind. Just pick out the first few you are considering.

Just some humble thoughts offered up for your consideration.

PS: To get 50 different posters might be a little too high of a number. I have been the organizer for postcard exchanges of varying types for a little while now. Considering the responses I have had for those, you might really want to consider lowering that number. At least to get started with. I would suggest 20-25 max. If all goes well with the first offering, how about raising that number by 5 each time. Start with 20 for the first offering. The second one 25, etc.